1. Field of the Invention
The present invention relates generally to integrated circuit switching power amplifiers, and more specifically, to a pulse-width modulated (PWM) power amplifier in which the pulse voltage and/or switching frequency are dependent on output signal amplitude.
2. Background of the Invention
Audio power output stage amplifiers consume the bulk of the wasted power in an audio system. In analog amplifiers, the difference between the voltage of the power supply rails and the voltage of the output signal is a voltage drop that determines the wasted power in combination with the instantaneous load current. Raising the efficiency of power amplifiers has typically been performed in one of two ways: either controlling the voltage of the power supply rails of a linear amplifier so that the power supply voltage is reduced when signal levels are low enough to permit such reduction, in so-called class-G and class-H amplifiers, or using switching power amplifiers in which output current is provided by pulses having only the duration needed to raise the output voltage to the necessary output level.
Pulse-width modulated (PWM) switching power amplifiers, referred to as class-D amplifiers, are currently in widespread use in automotive amplifiers and other audio amplifiers. Class D amplifiers are switching power amplifiers and as mentioned above, have higher efficiency than linear amplifiers, making them well suited for battery driven applications and applications where power dissipation in the form of heat is a problem, such as very high power professional audio applications, as in concert halls. However, the electromagnetic interference (EMI) generated by class-D amplifiers can be substantial, in contrast to that of linear amplifiers, in which generated EMI is almost nonexistent.
Because the output transducers driven by audio power amplifiers, e.g., speakers or headphones, provide filtering of the audio output, a class-D power amplifier can be connected directly to a transducer without a series output inductor, in configurations referred to as inductor-less designs. However, since the connection to a transducer is frequently a substantial length of wiring, and high frequency components of unfiltered switching pulses are therefore present on the wiring, the EMI generated by an inductor-less design is generally substantially greater than the EMI generated by designs including an inductor.
Therefore, it would be desirable to provide a switching power amplifier generating reduced EMI. It would further be desirable to provide an inductor-less switching power amplifier having reduced EMI.